The iron in a sample is in the form

The iron in a sample is in the form of an iron(II) compound, which is colorless because Fe2+ ions have zero molar absorptivity for visible light. Thus to determine the concentration of Fe2+ ions in solution using a visible wavelength of light, the ions must react quantitatively with other species to form colored compounds. Fe2+ ions react with
o-phenanthroline molecules, C12H8N2, to form complex of orange-red [Fe(o-phen)3]2+ ions